1. Field of the Invention
This invention relates to a process for the refining of chromium-containing molten steel wherein the recovery of Cr from chromium oxide in the slag, namely the reduction of the slag, and the removal of S from the molten steel, namely the desulfurization of the molten steel, are effected simultaneously and efficiently.
2. Description of the Prior Art
The conventional process for the refining of chromium-containing molten steel is divided, as illustrated in FIG. 1(a), into a step of decarburization, a step of reduction, and a step of desulfurization. During the decarburization, the molten steel is blown with O.sub.2 to strip C from the molten steel in the form of CO or CO.sub.2. At this time, part of Cr in the steel is released in the form of Cr oxide into the slag. The Cr oxide, therefore, is reduced by addition of Fe--Si as a reducing agent and CaO and CaF.sub.2 as slag-forming agents. The slag which has undergone this reduction, however, has a high melting point. For this and other reasons, it has no sufficient desulfurizing ability. It is customary for the conventional process to include the step of desulfurization wherein the slag just mentioned is discarded and new slag for desulfurization is prepared. This step entails drawbacks such as extension of the refining period, increase of the consumption of argon gas for refining, increase of the amount of refractories lost, and increase of the amount of flux for refining.
The CaO--SiO.sub.2 type slag has been adopted to date for the reduction and desulfurization of chromium-containing molten steel. In the operation, it has been customary for the basicity CaO/SiO.sub.2 to be selected in the range of 1.4 to 1.8 where the efficiency of reduction preponderates, or above 2.0 where the efficiency of desulfurization is more significant. This slag, however, has a very high melting point as noted from FIG. 2. Where the basicity CaO/SiO.sub.2 falls in the range of 1.4 to 1.8, the melting point of the slag reaches such a high level as 1700.degree. to 1900.degree. C. Actually, the slag additionally contains such components as MgO, Al.sub.2 O.sub.3, and TiO.sub.2 (whose total content barely falls in the range of 10 to 15%), which lower the slag's melting point. The lowered melting point of the slag still falls in the range of 1600.degree. to 1700.degree. C., a level which is high as compared with the level of 1580.degree. to 1650.degree. C. necessary for reduction and desulfurization of ordinary chromium-containing molten steel. For promoting the formation of slag, therefore, the elevation of the temperature of the molten steel or the addition of a large amount of CaF.sub.2 has been an inevitable recourse. These measures, however, notably aggravate loss of refractories of the refining furnace. Any attempt to curb the loss of refractories automatically results in retardation of reduction and desulfurization and in degradation of their efficiencies.
Japanese Patent Application Laid-open SHO 58(1983)-22318 discloses a method for reducing the time required for the refining of chromium-containing molten steel, which comprises adding to the slag, before completion of the decarburization, part or all of the amount of CaO required as a flux for desulfurization, and adding thereto, after completion of the decarburization, the remainder of CaO, if any, and the amount of Fe--Si required for reduction, thereby effecting the desulfurization simultaneously with the reduction. It can hardly be said, however, that this method gives a perfect solution to the aforementioned problems due to the use of the CaO--SiO.sub.2 type slag.
An object of this invention is to provide a process for the refining of chromium-containing molten steel which completely eliminates the aforementioned problems encountered by the conventional process of refining and, therefore, permits notable reduction of time required for the refining, improvement of the service life of the furnace, great saving of the consumption of slag-forming agent and refining gas, conspicuous improvement of the efficiency of desulfurization, and fair economization of energy.